The 22-year-old acknowledged that her body has changed since she began modeling at age 17, before she was diagnosed with Hashimoto's disease.

"Those of u who called me 'too big for the industry' were seeing inflammation & water retention due to that [condition]," she wrote. "Over the last few years I've been properly medicated to help symptoms including those, as well as extreme fatigue, metabolism issues, body's ability to retain heat, etc."

"I have always eaten the same, my body just handles it differently now that my health is better. I may be 'too skinny' for u, honestly this skinny isn't what I want to be, but I feel healthier internally and am still learning and growing with my body everyday, as everyone is," she wrote.

"I will not further explain the way my body looks, just as anyone, with a body type that doesnt suit ur 'beauty' expectation, shouldnt have to," Hadid concluded. "Stop putting me in that box just because u dont understand the way my body has matured."

The 22-year-old initially revealed her condition in December 2016 as part of a panel discussion for Reebok's Perfect Never campaign.

"My metabolism actually changed like crazy this year," she said during the panel. "I have Hashimoto's disease. It's a thyroid disease, and it's now been two years since taking the medication for it."

Gigi Hadid walks the runway wearing Jeremy Scott during New York Fashion Week in September 2015 versus February 2018. Frazer Harrison/Getty Images

So what, exactly, does this Hashimoto's disease do to the body? Here's a quick overview of the condition Hadid has been dealing with.

What is Hashimoto's disease?

A doctor examines a patient's thyroid. Shutterstock

Hashimoto's disease (also called Hashimoto's thyroidism) is an autoimmune condition, which broadly means that the body's immune system mistakenly attacks its own tissues. There are dozens of different types of autoimmune diseases, according to the NIH.

In Hashimoto's disease, the immune system makes antibodies that specifically attack and inflame the thyroid, a gland that sits at the base of your neck. The thyroid may be small, but it's massively important: It makes hormones that regulate metabolism by controlling breathing, heart rate, digestion, and body temperature.

What are the symptoms?

Hashimoto's disease usually progresses slowly over the years. Eventually, the thyroid stops working properly and doesn't produce enough of the hormones it's supposed to. This causes a number of unpleasant symptoms, including tiredness, increased sensitivity to cold, constipation, pale skin, brittle nails, hair loss, swelling of the tongue and a puffy face, unexplained weight gain, muscle aches and weakness, joint pain, long and heavy periods, depression, and lapses in memory.

How is it treated?

The good news is that the standard course of treatment— taking pills that contain a man-made version of thyroid hormones — can reverse the symptoms of the disease. (In Hadid's case, they appear to be working — after more than three years on her medication, she has looked strong and healthy modeling designers' Fall/Winter 2018 collections.)

Starting treatment as soon as possible is crucial, by the way: If Hashimoto's is left untreated, a whole bunch of other medical issues can develop, including goiter, dangerously high cholesterol, heart problems, mental health issues, and even a scary life-threatening coma called myxedema. Untreated Hashimoto's disease in mothers can also affect the development of babies.

Think you might have a thyroid problem?

Thyroid issues are more common than you might think. The NIH says about 1 in 100 Americans has an overactive thyroid (that's the opposite of what's happening in Hashimoto's) while around 1 in 20 have an underactive thyroid, whether that's caused by Hashimoto's or something else.